The subject matter of the present invention relates to an apparatus and method for analysis of received seismic data for the purpose of reducing bias in said seismic data and to accurately determine the location of events, such as peaks and troughs and zero cross crossings, in each of the plurality of seismic traces which comprise the seismic data.
Petroleum exploration professionals are required to have a detailed understanding of subsurface geology and the geometry of subsurface structures. Particularly, it is important for petroleum exploration professionals to accurately locate a horizon or interface in the subsurface geological structure. Received seismic data is analyzed for the purpose of locating the horizon or the interface in the subsurface geological structure.
Seismic data is displayed using a plot of common attributes, such as amplitudes of seismic reflections from subsurface structures (i.e., horizons). For example, a plot of amplitudes of seismic reflection data as a function of time or depth would constitute one seismic trace, and a multitude of such seismic traces lying in a first plane would represent one surface of a 3D cube in an earth formation (see FIG. 5). In addition, a multitude of such seismic traces lying in a plurality of other additional planes disposed adjacent to the first plane would represent the entire 3D cube in the earth formation. In the first plane, if a line were drawn (on a workstation display by an operator sitting at a workstation) transversely across the plane starting at a location which would correspond to a horizon, that line could represent an edge of a horizon provided, however, that the line falls accurately on the events (such as the "peaks") of each of the seismic traces which lie on or near that line in the first plane. However, if that line does not fall on the event (such as the "peaks") of each of the seismic traces which lie on or near that line in the first plane, that line would not accurately represent an edge of the horizon. Furthermore, if other additional lines were drawn transversely across the other additional planes starting at locations which would correspond to a horizon, those other additional lines would collectively define the horizon itself provided, however, that the other additional lines fall accurately on the events (such as the "peaks") of the seismic traces which lie on or near the other additional lines in those other additional planes. However, if the other additional lines do not fall accurately on the events (i.e. the peaks) of the seismic traces which lie on or near the other additional lines in the other additional planes, the horizon itself cannot be accurately defined or represented. This process, practiced by the operator at the workstation, of drawing lines across a plane which contains a plurality of seismic traces starting at a location which would correspond to a horizon is known as "picking" and the process of disposing that line on the events (such as the peaks or the troughs or the zero crossings) of the seismic traces which fall on or near that line is called "snapping". Thus, the term "snapping to the peak" refers to first drawing a line across a plane containing a plurality of seismic traces (starting at a location which would correspond to a horizon) and then pressing a key on a computer keyboard which will move or "snap" that line onto the events (such as the peaks or the troughs or the zero crossings) of the seismic traces which lie on or near that line on the plane. The aforementioned "snapping to the peak" concept will be better understood in the following discussion with reference to FIGS. 14a and 14b.
Therefore, in order to accurately define each horizon in an earth formation, it is very important that the location of each of the events, such as the peaks and troughs and zero crossings, of each of the seismic traces (lying in each of the planes of a 3D cubic volume of an earth formation) be accurately determined. The location of an "event" (such as a peak or trough or zero crossing) is defined by its input time and its amplitude at that input time.
However, prior art techniques which involve "snapping to the peak" were biased in favor of the individual samples of input seismic data. That is, when a line is drawn across a plane of the 3D cubic volume, and when the key on the keyboard was pressed to "snap" that line onto an event, such as the peaks, of the seismic traces which lie on or near that line in the plane, the prior art "snapping" technique (which is adapted for determining the location of the events in the seismic traces) would determine that the events, such as the peaks, were located near an adjacent sample; that is, the location of the events were "biased" by the prior art "snapping technique" toward the nearest, adjacent seismic samples (see FIG. 8), thereby creating a "biasing error". This "biasing error" means that the line drawn across the plane of seismic traces, by a workstation operator, would not accurately fall on the edge of a horizon in the 3D cubic volume of earth formation. In three dimensional terms, this "biasing error", created by the prior art "snapping technique", means that the horizon itself would not be accurately defined. As noted earlier, petroleum exploration professionals must know the accurate location of each horizon in an earth formation in order to further know the location of underground deposits of hydrocarbons in the earth formation for subsequent drilling purposes.
Recent disclosures which have made an effort to solve the horizon "picking" problems are, for example: U.S. Pat. No. 5,056,066 to Howard entitled "A method for attribute tracking in seismic data"; U.S. Pat. No. 5,570,106 to Viswanathan entitled "Method and Apparatus for creating horizons from numerical 3-D seismic data"; U.S. Pat. No. 5,537,365 to Sitoh entitled "Apparatus and Method for evaluation of picking horizons in 3D seismic data"; U.S. Pat. No. 5,251,184 to Hildebrand et al entitled "Method and Apparatus for finding horizons in 3D seismic data"; and U.S. Pat. No. 5,153,858 to Hildebrand entitled "Method for finding horizons in 3D seismic data".